Mountain Instruments MI-250 questions

[strongmanmike (Michael)]

Originally Posted by baileys2611

Thanks Mike. By the way, Ron Abbott from Land of Oz Observatory mentioned your name recently and said to say hi.

Re your question - Gah! Maths! Here goes:

SBIG ST-i is 7.4 microns, I'm using off-axis guiding so the scope focal length comes into it. The scope is 2032mm f/8 (I like it a lot!).
I was binning at 2x2 during this guiding.

Based on the formula here: I get a calculation of 1.5 arcseconds per pixel. So, the graph would show an average error of 0.7 arcseconds error in RA and 0.5 in Dec I think?

Ok well that's certainly not terrible

BTW how would you rate the overall astro conditions in the Murrumbateman area? ie. seeing, sky darkness, frequency of fog etc?

Cheers

Mike

[Atmos (Colin)]

Originally Posted by baileys2611

Thanks Mike. By the way, Ron Abbott from Land of Oz Observatory mentioned your name recently and said to say hi.

Re your question - Gah! Maths! Here goes:

SBIG ST-i is 7.4 microns, I'm using off-axis guiding so the scope focal length comes into it. The scope is 2032mm f/8 (I like it a lot!).
I was binning at 2x2 during this guiding.

Based on the formula here: I get a calculation of 1.5 arcseconds per pixel. So, the graph would show an average error of 0.7 arcseconds error in RA and 0.5 in Dec I think?

That actually seems pretty good!
There were times I could get that with the EQ6 (for short periods) but then there would be a random spike here and there that would mess things up. And it would only happen on nights of good steady seeing!

If you're getting that level of guiding without any random spikes then it's pretty good.

[PRejto (Peter)]

Simon,

I agree with an earlier comment that the errors you are seeing might be due to seeing given RA and DEC look similar. I think you could judge this by looking at the guiding graphs while not sending guide corrections. Since DEC isn't moving everything you see in terms of excursions would be due to seeing whereas RA would be PE + seeing. Another tool worth experimenting with is the Guiding Assistant in PHD2. It basically does the above and analyses the result making suggestions for exposure duration and min move settings. It also has a tool to measure DEC backlash. On Paramounts the backlash tool doesn't really work (It seems to measure stiction) but it might prove useful with your mount and could quantify adjustments.

Peter

[baileys2611 (Simon Bailey)]

Originally Posted by strongmanmike

Ok well that's certainly not terrible

BTW how would you rate the overall astro conditions in the Murrumbateman area? ie. seeing, sky darkness, frequency of fog etc?

Cheers

Mike

Mike, summary for you because I think I've rambled on a bit below: the overall conditions have been 'good' - but not 'great' and that's changed recently. About three years ago it was clear days and cold clear nights in winter, very little humidity or moisture. Lately it's hard to pick.

Some detail
We have great dark skies, very little to no light pollution unless you are pointing south and low on the horizon - that's where Canberra sits and you get some interference there. I can see the large and small Magellanic Clouds on most dark nights (when there's little to no moon and I don't have superman eyes).

Seeing varies because of a few factors, the relative humidity is high in Spring and the upper atmosphere wind conditions are different to those near the ground - we often have a pretty high jetsteam. I use this site to gauge upper atmosphere conditions and temper that with those on the ground.

Fog happens but normally in the mornings. The weather has changed a bit recently, it used to be many mornings in winter, and was often a 'pea souper' but lately it's harder to pick. Nights - not so much, if it's a clear day it's normally a clear night and winter in Canberra (and Murrumbateman) has normally been clear days and cold nights.

Poor Paul in Adelaide soaks up a lot of the moisture before it comes here, we often get the 'leftover' weather from Adelaide in winter. In summer an easterly comes in from the coast and brings the moisture with it. Spring, we're getting jetstreams off the south so it can be frustrating and Autumn is normally good results.

[baileys2611 (Simon Bailey)]

Originally Posted by PRejto

Simon,

I agree with an earlier comment that the errors you are seeing might be due to seeing given RA and DEC look similar. I think you could judge this by looking at the guiding graphs while not sending guide corrections. Since DEC isn't moving everything you see in terms of excursions would be due to seeing whereas RA would be PE + seeing. Another tool worth experimenting with is the Guiding Assistant in PHD2. It basically does the above and analyses the result making suggestions for exposure duration and min move settings. It also has a tool to measure DEC backlash. On Paramounts the backlash tool doesn't really work (It seems to measure stiction) but it might prove useful with your mount and could quantify adjustments.

Peter

Excellent. I need to do some reading and a bit more research on PHD2 I think - I get what you've said at a broad level and it definitely makes sense but I'll need to practice a little with PHD2 to understand it's output - haven't used it much (just a little).

[Octane (Humayun)]

Sorry to hear of your mount woes, Simon. Hopefully you get it all ironed out for the summer objects.

Just wanted to thank you for the link to that excellent Meteoblue web site. Wow!

H

[baileys2611 (Simon Bailey)]

Originally Posted by Octane

Sorry to hear of your mount woes, Simon. Hopefully you get it all ironed out for the summer objects.

Just wanted to thank you for the link to that excellent Meteoblue web site. Wow!

H

Ah, thanks Humayun. It's all good, a long(ish) life and opportunities aplenty . No problems re the site, it's pretty good I used to use Sky Harbinger (got pointed to it on the IceInSpace forums) but alas, things change. Sunday night is looking good!

[gregbradley]

Originally Posted by baileys2611

Thanks Mike. By the way, Ron Abbott from Land of Oz Observatory mentioned your name recently and said to say hi.

Re your question - Gah! Maths! Here goes:

SBIG ST-i is 7.4 microns, I'm using off-axis guiding so the scope focal length comes into it. The scope is 2032mm f/8 (I like it a lot!).
I was binning at 2x2 during this guiding.

Based on the formula here: I get a calculation of 1.5 arcseconds per pixel. So, the graph would show an average error of 0.7 arcseconds error in RA and 0.5 in Dec I think?

Something must be not right here.
That sort of guiding is impossible with anything less than the absolute very best. PMX, PME specifications are 7 arc seconds of error. AP mounts are similar perhaps a bit tighter.

If you had guiding anywhere approaching that sort of accuracy you would have perfect round stars in any length of exposures.

PE of most high end mounts would be around 3-6 arc seconds. MI 250 has the diamond grease smoothed out worms so it may be around the 5 arc second band.

But if you are having some sort of mechanical problem it would be nowhere near that.

I don't quite trust PHD2 graphs. I have used them before that showed awesome guiding yet the subexposures had eggy stars. Not sure why that was but it did.

If you need more advice about your MI250 you should give Larry at MI a call. He's a pretty helpful guy or send him an email. I don't know if they are still in business though.

Greg.

[baileys2611 (Simon Bailey)]

Originally Posted by gregbradley

Something must be not right here.
That sort of guiding is impossible with anything less than the absolute very best. PMX, PME specifications are 7 arc seconds of error. AP mounts are similar perhaps a bit tighter.

If you had guiding anywhere approaching that sort of accuracy you would have perfect round stars in any length of exposures.

PE of most high end mounts would be around 3-6 arc seconds. MI 250 has the diamond grease smoothed out worms so it may be around the 5 arc second band.

But if you are having some sort of mechanical problem it would be nowhere near that.

I don't quite trust PHD2 graphs. I have used them before that showed awesome guiding yet the subexposures had eggy stars. Not sure why that was but it did.

If you need more advice about your MI250 you should give Larry at MI a call. He's a pretty helpful guy or send him an email. I don't know if they are still in business though.

Greg.

I thought that too Greg. This is after PEC is applied. I cannot remember what the error was before PEC (a figure like +-12 comes to mind) - but I looked at the calcs and wondered about how this was possible also.

[Octane (Humayun)]

Eggy stars could also be due to a slight imbalance? I know I've had that. Even with my little setup while self-guiding. MaxIm DL guide graph was quite flat but the stars were elongated. Checked my balance and I was out a fair bit.

H

[GC - South Aus]

G'day Simon

Been reading this thread with great interest.

The MI-250 is a beautiful piece of machinery, it was the mount I always wanted (dreamed about - scary) and even looked at freighting one from the US a few years back from a CNC post. But, freight costs were prohibitive, as was my wife!

Hope you get it all worked out, I am sure it will serve you well when you do!

Regards

Gav

[strongmanmike (Michael)]

Originally Posted by baileys2611

Mike, summary for you because I think I've rambled on a bit below: the overall conditions have been 'good' - but not 'great' and that's changed recently. About three years ago it was clear days and cold clear nights in winter, very little humidity or moisture. Lately it's hard to pick.

Some detail
We have great dark skies, very little to no light pollution unless you are pointing south and low on the horizon - that's where Canberra sits and you get some interference there. I can see the large and small Magellanic Clouds on most dark nights (when there's little to no moon and I don't have superman eyes).

Seeing varies because of a few factors, the relative humidity is high in Spring and the upper atmosphere wind conditions are different to those near the ground - we often have a pretty high jetsteam. I use this site to gauge upper atmosphere conditions and temper that with those on the ground.

Fog happens but normally in the mornings. The weather has changed a bit recently, it used to be many mornings in winter, and was often a 'pea souper' but lately it's harder to pick. Nights - not so much, if it's a clear day it's normally a clear night and winter in Canberra (and Murrumbateman) has normally been clear days and cold nights.

Poor Paul in Adelaide soaks up a lot of the moisture before it comes here, we often get the 'leftover' weather from Adelaide in winter. In summer an easterly comes in from the coast and brings the moisture with it. Spring, we're getting jetstreams off the south so it can be frustrating and Autumn is normally good results.

Thanks a lot for that Simon. We are contemplating a move back out to a rural location near Canberra and obviously Murrumbateman is an option. I have had feedback from Anthony Wesley and Dave Herald already and you all seem to confirm that the main potential problematic issue is fog..? The development of Fairly could be problematic in the future too, so that would need to be avoided and will likely degrade the quality of the night sky as time goes on if it were to in anyway mimic Googong and Bungendore....never the less the area is probably a good compromise.

Thanks for that seeing site, hadn't seen it before. My last OT question, have you found it to be relatively accurate?

Mike

[gregbradley]

Originally Posted by baileys2611

I thought that too Greg. This is after PEC is applied. I cannot remember what the error was before PEC (a figure like +-12 comes to mind) - but I looked at the calcs and wondered about how this was possible also.

Even with PEC applied it sounds highly unlikely in my experience. The proof would be in the images. If you are getting perfectly round stars in long exposures then you know the PE is very low. If you are not then the numbers are not correct.

Greg.

[gregbradley]

Originally Posted by baileys2611

I thought that too Greg. This is after PEC is applied. I cannot remember what the error was before PEC (a figure like +-12 comes to mind) - but I looked at the calcs and wondered about how this was possible also.

Originally Posted by strongmanmike

Thanks a lot for that Simon. We are contemplating a move back out to a rural location near Canberra and obviously Murrumbateman is an option. I have had feedback from Anthony Wesley and Dave Herald already and you all seem to confirm that the main potential problematic issue is fog..? The development of Fairly could be problematic in the future too, so that would need to be avoided and will likely degrade the quality of the night sky as time goes on if it were to in anyway mimic Googong and Bungendore....never the less the area is probably a good compromise.

Thanks for that seeing site, hadn't seen it before. My last OT question, have you found it to be relatively accurate?

Mike

I noticed mist and fog in some areas was common around Goulburn in the cooler months. The solution to fog is being on the top of a hill. I have never been fogged out at Bigga even when there has been fog in lower lying areas.

Greg.

[baileys2611 (Simon Bailey)]

Originally Posted by Octane

Eggy stars could also be due to a slight imbalance? I know I've had that. Even with my little setup while self-guiding. MaxIm DL guide graph was quite flat but the stars were elongated. Checked my balance and I was out a fair bit.

H

Yeah, I might need to check that. I did do the whole balance thing until you can move it with your pinky and it stays there, in a few different directions. Problem with the MI-250 is that it's actually quite tight, even with the ... things ... knobs that tighten stuff... forgotten their names... those things unscrewed a lot. I wish this had a balance function similar to the HEQs or Ioptrons. I've read that if you move it manually, it throws out T-point out because the encoders aren't very accurate.

So, I'll try re-balancing and do another t-point after that. Cannot hurt anyway, it's pointing accuracy is down to 50 arc-seconds so it will be good to see if that's repeatable.

I'll also get a tak I think and check the collimation and will need to read a little bit on making sure that the optical train is aligned nicely. I'm not getting significant astigmatisms that I can see, but I think I'll take a look through a tak scope and see what comes of it.

[baileys2611 (Simon Bailey)]

Originally Posted by strongmanmike

Thanks a lot for that Simon. We are contemplating a move back out to a rural location near Canberra and obviously Murrumbateman is an option. I have had feedback from Anthony Wesley and Dave Herald already and you all seem to confirm that the main potential problematic issue is fog..? The development of Fairly could be problematic in the future too, so that would need to be avoided and will likely degrade the quality of the night sky as time goes on if it were to in anyway mimic Googong and Bungendore....never the less the area is probably a good compromise.

Thanks for that seeing site, hadn't seen it before. My last OT question, have you found it to be relatively accurate?

Mike

Fog is more of an issue now than it used to be - but that's also changed in the last 10 years. I've lived in and around Canberra all my life and when we were in an el-nino we had better clear weather (higher pressure). Over the last 10 to 12 years that's changed as the humidity has been higher. I wonder if that's a permanent future?

Light pollution will be interesting when Fairly is in full swing. The estate's western border is right up against my eastern, but I haven't noticed much change yet even though the street lights are all on now.

Sorry Mike - what does OT mean?

[baileys2611 (Simon Bailey)]

Originally Posted by gregbradley

Even with PEC applied it sounds highly unlikely in my experience. The proof would be in the images. If you are getting perfectly round stars in long exposures then you know the PE is very low. If you are not then the numbers are not correct.

Greg.

Last stub I took was 2400 seconds and the stars were lovely Then the one after that was a bit eggy, I can't pick it.

[gregbradley]

Originally Posted by baileys2611

Last stub I took was 2400 seconds and the stars were lovely Then the one after that was a bit eggy, I can't pick it.

That's a bit trickier when you get good guiding then poor. I have had that happen.

Apart from mechanical issues with the mount such as some slop in the bearings or the worm is not at the correct pressure against the gears it could be other factors

1. Balance of OTA at the angle you image at. That is if you have loaded gear on top of your OTA like guide scopes etc that are heavy and above the centre of gravity it can balance when horizontal but be quite imbalanced at 60 degrees.

2. Cable drag. A common hidden cause of erratic guiding.
3. I recalibrate when I do a meridian flip.
4. First thing I do if I get poor guiding is pick another guide star. Its amazing how much better the guiding can suddenly become. Double stars are sometimes hard to notice until you start guiding on them.
5. Wrong autoguiding settings. You can have too short an exposure for autoguiding on high end mounts. The weaker the mount probably the shorter the guide exposure. On the weekend I noticed I got better guiding do a bit longer on the guide exposures with my AP1600. That's because natively it has such little PE. If your PE figures are even close to what you quoted then longer guide exposures may be the go.

Using the right min/max settings, the right aggressiveness settings.
SkyX now allows you to alter X Y plus and minus for each aggressiveness. A bit trial and error but often one gets worse errors than the other and you can attempt to improve that.

6. Spot on polar alignment is the ultimate key. Also if you have been monkeying with the bearings etc you would need to redo your PEC as the older one would no longer be valid.

Sky X accurate polar alignment routine with TPoint is the best method I have come across for truly excellent polar alignment. Every sophistication seems to be built into it.

Greg.

[baileys2611 (Simon Bailey)]

Hi Everyone,

Thanks for all your input. It's been an education. Worth the price? Not sure...but then it's been good to understand the mechanics in detail and all the factors in play.

My last shot at this had me taking photos all night (automated of course) and had some good results, some odd ones but I put that down to cables, the camera and niggling things that happen with tracking from time to time (hot pixels...I hate them!!).

Anyway, it's been a good experiment and something to learn from. I think I'm going to look at upgrading however to something that is more suited to what I want.

Having re-done some of the mechanics, it is probably below spec by a small fraction but is now much closer than it was when I received it. Having said that, I wouldn't recommend it for remote operation long duration astrophotography - which I like to do.

If I'm going to pass this on to anyone, it would be more suited to visual astronomy or wide field and fast photography.

I thought I would post this here though to get a feeling for if this is reasonable to look at selling. Given the history and work I've done on it - would anyone benefit from buying it or would it be best for me to shelve it and call it a good learning experience?

I know the answer may be 'put it in the equipment sale forum, be open and honest about the history and see if there are any takers' which is logical - but am after a sanity check to see if this is actually reasonable in the first instance. If it's not, I won't consider it.